These studies have demonstrated that SV40 T Ag complexes with the host-encoded 48K phosphoprotein in transformed cells. An A locus mutant of the virus which cannot transform also cannot either cause the expression of the 48K HTA or permit the formation of the T Ag.48K HTA complex, suggesting that the complex stabilizes the 48K HTA allowing its detection. Differential phosphorylation of large T Ag evidently modulates the complex formation, as the 48K HTA binds only to the highly phosphorylated form of T Ag. Because the 48K HTA has been found at an elevated level in cells transformed by a variety of carcinogens in addition to SV40 (MSV, Abelson Leukemia virus, methylcholanthrene, "spontaneous events," etc.) it is likely that elucidation of its interaction with the SV40 transforming product T Ag with the 48K HTA, especially to determine whether the particular amino acids which are phosphorylated are important to the interaction. 1. We will also determine whether the tsA-effect is reversible. 2. We will determine whether the highly phosphorylated, 48K-bound T Ag has a different nucleotide binding specificity or affinity than the underphosphorylated, free form. 3. Using the new system of Deppert (J. Virol. 35, 505 (1980)) we will determine whether we can visualize T Ag and the 48K HTA in the plasma membrane. 4. We will determine the kinetics of the phosphorylation of T Ag, the complex formation, and transport to plasma membrane and nucleus in vivo.